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I think it is best to do a mix of sitting and standing. It is not that sitting is terribly evil, it is necessary for a rest from time to time. The problem is really more about being sedentary. For me it is best to at least find a position either seated or standing (or leaning on a stool) where at least your book is in a position where you don’t have to flex your neck and look down for extended periods of time. Just make sure to always be changing positions and places you read every 30-45 minutes. I like to take timed breaks where I do a couch stretch, hollow rocks, or some medium weight KB swings for a few minutes.